Mounting system for transducer

ABSTRACT

A mounting system for mounting a transducer to a case, such that the transducer position is fixed and stable, and such that stresses on the transducer due to thermal expansion are minimized. The mounting system comprises a band, lower arms extending in a first axial direction from the band, and upper arms extending in a second, opposite axial direction from the band. The transducer arms are connected to the lower, and the upper arms to the case. In response to differential thermal expansion, the arms S bend in a radial direction.

LICENSE RIGHTS

The U.S. government has certain rights in this invention, as providedfor by the terms of Contract No. 04704-86-C-0160 awarded by theDepartment of the Air Force.

TECHNICAL FIELD

The present invention relates to mounting systems for precisiontransducers and, in particular, to a stress-free mounting system for atransducer such as an accelerometer.

BACKGROUND OF THE INVENTION

It is often necessary to isolate a precision transducer from externalstress. Such stress may be caused by mechanical distortion of the case(or other support structure) to which the transducer is mounted, or bydifferential thermal expansion or contraction between the transducer andthe case. Isolation from external stress can in principle be achieved byusing a compliant mounting system. However, a compliant mounting systemwill not in general provide precise and stable alignment of thetransducer with respect to its case. For many transducers, (e.g.,accelerometers), such alignment is critical for achieving properoperation. A compliant mounting system may also result in unwantedmechanical oscillation of the transducer when the case is exposed tovibration.

One prior accelerometer mounting technique has been to connect theaccelerometer to the case by means of a metal ring, or by means of astructural adhesive such as an epoxy resin. The prior noncompliantmounting techniques result in stress being transmitted to theaccelerometer due to differential thermal expansion between theaccelerometer and the mounting ring and case. These prior techniquesalso transmit stress to the accelerometer when the case is subjected tomechanical distortion. Distortion of the case be induced by mounting thecase to an external structure, or by differential thermal expansionbetween the case and the external structure. Such stress may lead tothermally induced errors in the accelerometer output.

A second type of prior accelerometer mounting system is described in PCTpatent application No. WO 88/00348. This system utilizes a devicecomprising a plurality of mounting elements joined by bridge sections toform a ring from which the mounting elements extend in an axialdirection. In a first embodiment, the mounting elements include endsthat are connected to the case, while the ring itself is directly joinedto the transducer. Although this technique provides a stable mount witha certain degree of relief from thermal strain, the direct mounting ofthe ring to the transducer nevertheless produces thermally inducedstrains whenever the mounting device and transducer are fabricated frommaterials having different thermal expansion properties. In a secondembodiment, the mounting elements are divided into two interleaved sets,with one set being connected to the transducer, and the other to thecase. While this approach avoids the direct bonding of the ring to thetransducer, it effectively divides the number of support points by two,therefore, providing a less rigid mount.

SUMMARY OF THE INVENTION

The present invention provides an improved system for mounting atransducer to a case or other support. The invention permits thetransducer to be secured in a fixed and stable position within the case,and minimizes the transmission of stresses to the transducer resultingfrom differential thermal expansion.

The mounting system comprises a band, and a plurality of lower armsextending in a first direction from the band. The first direction isparallel to a longitudinal axis about which the band extends. Each lowerarm is relatively compliant for bending in a radial direction normal tothe longitudinal axis, and relatively noncompliant for bending in otherdirections. A plurality of upper arms extend in a second, oppositedirection from the band. The upper arms are relatively compliant forbending in the radial direction, and relatively noncompliant for bendingin other directions. Transducer attachment means spaced from the band inthe first direction permit attachment of the transducer and the lowerarms to one another. Similarly, support attachment means spaced from theband in the second direction permit attachment of the support and theupper arms to one another.

In one preferred embodiment, the band is dimensioned such that thetransducer can be positioned within the band without contacting theband. The transducer attachment means may comprises an inwardlyextending flange on each lower arm, and the support attachment means maycomprise an outwardly extending flange on each upper arm. The bandpreferably comprises a material having a coefficient of thermalexpansion between that of the transducer and that of the support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut away perspective view showing an accelerometermounted within a case using a mounting device according to the presentinvention;

FIG. 2 is a perspective view of a preferred embodiment of the mountingdevice;

FIG. 3 is a partial cross-sectional view of the accelerometer, mountingdevice, and case; and

FIG. 4 is a partial cross-sectional view of a second embodiment of themounting device.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the mounting system of the present inventionis illustrated in FIGS. 1-3. As shown in FIG. 1, the system comprisesmounting device 12 that is used to mount accelerometer 14, or a similartransducer, within case 16. The accelerometer and case are bothcylindrical in shape. The accelerometer has a sensing axis SA alongwhich it is sensitive to acceleration, the sensing axis being parallelto the central longitudinal axis of the accelerometer. The case isdimensioned such that the accelerometer and mounting device can bepositioned within the case, as shown in FIGS. 1 and 3. Generally, case16 includes mounting flange 18 that is used to mount the case to anappropriate external structure. Flange 18 is omitted in FIG. 3, tosimplify the illustration

Mounting device 12 comprises a cylindrical central strip that comprisesa band 20, a plurality of upper arms 22 extending upwardly from theband, and a plurality of lower arms 24 extending downwardly from theband. It will be appreciated that the terms "upper" and "lower" have noabsolute significance, and are simply used to clarify reference to theFIGURES. As most clearly shown in FIG. 3, upper arms 22 are used tomount mounting device 12 to case 16, while lower arms 24 are used tomount accelerometer 14 to the mounting device.

In the illustrated embodiment, upper end 30 of each upper arm 22includes outwardly projecting flange 32 that sits in a shoulder 34formed in case 16. The width (i.e., the radial direction in FIG. 3) offlange 32 is greater than the width of shoulder 34, such that upper arm22 is spaced from case 16. Lower end 40 of lower arm 24 includes flange42 that is shaped so as to form positioning surface 44 and seatingsurface 46. Seating surface 46 is used to support accelerometer 14 at anappropriate vertical position within the case, while positioning surface44 is used to radially position accelerometer 14 within mounting device12, and to space the accelerometer from lower arm 24, such that thelower arm does not contact the accelerometer.

Mounting, device 12 provides precise and stable alignment ofaccelerometer 14 with respect to case 16, such that accelerometerundergoes minimal translational or rotational movement with respect tothe case. However, the mounting device does permit differential radialand/or volumetric thermal expansion or contraction between theaccelerometer and the case, and also serves to isolate the accelerometerfrom stresses that would otherwise result from distortion of the case.Case distortion may be caused by securing mounting flange 18 to asurface that is not perfectly flat, or by differential thermal expansionbetween the mounting flange and the external structure to which it isattached.

Each of the upper and lower arms is dimensioned such that it iscompliant to S-bending in a radial direction, but is substantially rigidwith respect to circumferential or axial deflection. Thus, differentialradial thermal expansion or contraction between the accelerometer andthe case causes each arm to flex in its compliant mode, to take up thedifferential movement without transmitting stress to the accelerometer.However, the rigidity of the arms in the other directions results in amounting system in which the accelerometer undergoes minimal rotationalor translational movement with respect to the case. The compliance toS-bending in the radial direction is achieved by making the width(circumferential dimension) and length (axial direction) of each armsubstantially greater than its thickness (radial dimension). The widthof each arm should, of course, be limited with respect to thecircumference of the mounting device, such that each arm is essentiallyplanar and compliant in the radial direction. The distance betweenadjacent arms should be large enough to avoid interference between thearms due to thermal expansion or acceleration inputs.

The mounting system of the present invention provides an arrangement inwhich the transducer and the case are attached to arms to opposite sidesof the band. Thus, as a part attached to either the upper or lower armsexpands, due to a temperature change, the arms S-bend radially outward,restrained by the band. Such arm deflections result in a symmetricalradial load in the band, which is restrained by band tension. This load,combined with expansion due to the thermal coefficient of expansion ofthe band itself, results in a very small expansion of the band,typically 10 percent of the original expansion at the ends of the arms.In the prior art device in which the band is connected directly to thetransducer, this band expansion would be applied as direct strain to thetransducer itself. The mounting system of the present inventiontransmits this band strain through a second set of S-bending arms. Thistransducer the band strain into a very light load on the transducer andcase. For a typical application, the resulting strain induced by a giventhermal expansion can be reduced by a factor of 40 over that produced ina similar application using such a prior mounting device.

In a typical application for mounting an accelerometer, case 16 isfabricated from stainless stell and has a coefficient of thermalexpansion on the order of 18 ppm/°C., the mounting device is composed ofInvar and has a coefficient of 1.5 ppm/°C., and the accelerometercomprises fused quartz and has a coefficient of 0.5 ppm/°C. With theband mounted directly to the accelerometer, the coefficient mismatchwould result in a significant amount of stress on the accelerometer.However, using the present invention, these stresses are attenuated byS-bending of the arms. In a typical application, the change of size ofthe band in response to a temperature change would occur primarily dueto the coefficient of thermal expansion of the band itself rather thanto stress transmitted to the band from the transducer or case throughthe S-bending arms.

In the mounting device shown in FIG. 1, the transducer and case areapproximately coextensive along their longitudinal axes. However, themounting device can be used in other configurations. For example, FIG. 4shows an arrangement in which transducer 50 is mounted to case 52 bymounting device 54. Mounting device 54 comprises band 56, upper arms 58extending upward from the band, and lower arms 60 extending downwardfrom the band. Transducer 50 is connected directly to the inner surfacesof the upper portions of upper arms 58, whereas the outer surfaces ofthe lower ends of lower arms 60 are secured directly to shoulder 62 ofcase 52. This arrangement produces all the advantages described abovefor the embodiment of FIGS. 1-3, except that the overall configurationis less compact.

While the preferred embodiments of the invention have been illustratedand described, variations will be apparant to those skilled in the art.For example, structures equivalent to flanges 32 and 42 could be formedas parts of the case and transducer, respectively, rather than as partsof the mounting device.

Accordingly, the scope of the invention is to be determined by referenceto the following claims.

The embodiments of the invention in which an exclusive property ofprivilege is claimed are defined as follows:
 1. A mounting system formounting a transducer to a support, the mounting system comprising:aband; a plurality of lower arms extending in a first direction from theband, the first direction being parallel to a longitudinal axis aboutwhich the band extends, each lower arm being relatively compliant forbending in a radial direction normal to the longitudinal axis andrelatively noncompliant for bending in other directions; transducerattachment means spaced from the band in the first direction forpermitting attachment of the transducer and the lower arms to oneanother; a plurality of upper arms extending in a second direction fromthe band, the second direction being opposite to the first direction,each upper arm being relatively compliant for bending in the radialdirection and relatively noncompliant for bending in other directions;and support attachment means spaced from the band in the seconddirection for permitting attachment of the support and the upper arms toone another.
 2. The mounting system of claim 1, wherein for each lowerarm there is associated upper arm connected to the band at the sameposition around a circumference of the band as the lower arm.
 3. Themounting system of claim 1, wherein the support attachment meanscomprises an outwardly extending flange on each upper arm.
 4. Themounting system of claim 1, wherein the transducer attachment meanscomprises an inwardly extending flange on each lower arm. each inwardlyextending flange including intersecting seating and positioning surfacesfor mounting the transducer, the seating surface being parallel to saidplane in which the band lies, and the positioning surface being parallelto the lower arm.
 5. The mounting system of claim 1, wherein thetransducer and support have different coefficients of thermal expansion,and wherein the band comprises a material having a coefficient ofthermal expansion between that of the transducer and that of thesupport.
 6. The mounting system of claim 1, wherein the band isdimensioned such that the transducer can be positioned within the bandwithout contacting the band.
 7. The mounting system of claim 6, whereinthe transducer attachment means comprises an inwardly extending flangeon each lower arm, each inwardly extending flange including intersectingseating and positioning surfaces for mounting the transducer, theseating surface being parallel to said plane in which the band lies, andthe positioning surface being parallel to the lower arm.
 8. The mountingsystem of claim 1, wherein each lower arm includes a distal end spacedfrom the band in the first direction, wherein the transducer attachmentmeans comprises an inner surface of the distal end of each lower arm,and wherein the mounting system is dimensioned such that the transducercan be directly attached to said inner surfaces.